How do I help a friend dealing with illness and depression?

It's possible my close friend is lying to me about her cancer condition..or is just plainly out to kill herself. She's in her late 30's and had a lump removed above her shoulder blade..about the size of a medium orange. It grew back. I have offered several times to accompany her on a doctor visit (she's cried that there is no one who cares enough to go with her), she inevitably cancels and "reschedules" without alerting me until after. She says she is now undergoing radiation therapy...and is approximately 1-2 weeks from the end of treatment. 1) This radiation therapy is targeting the area where the tumor was removed; yet she wears her purse straps over the area without issue. 2) She basks in the sun several times a week...??? 3) I never get a straight answer from her as to the exact type/diagnosis of this particular cancer 3) She doesn't appear to have any of the 'normal' side-effects. her appetite remains good...she drinks alcoholic beverages (2-3 daily). and continues to abuse her body with things like narcotics and prescription drugs 4) She tells me her white blood count is very low and "they're" concerned. They want to increase the frequency of her radiation treatments?? Bottom line? I'm VERY concerned here...she's EXTREMELY depressed and yet will not address the need for help with her doctor on this issue. I believe she has several addictions issues and yet, blood testing doesn't reveal this to her doctor? She can be very angry and cut off anyone who questions her, being defensive and abusive. I'm her only friend. I love her, and this is draining me horribly. HOW DO I HELP HER? How do I even know if this is truly happening to her and NOT some ploy as a cry for help? Please advise me...PLEASE.